Longitudinally coupled resonator type surface acoustic wave filter and communication apparatus incorporating the same

ABSTRACT

A longitudinally coupled resonator type surface acoustic wave filter having a balance-unbalance conversion function achieves improved amplitude balance and phase balance. The surface acoustic wave filter includes first, second and third IDTs. The second IDT is positioned in the center of the three IDTs and has an even number of electrode fingers. The polarity of the electrode finger of the first IDT adjacent to the second IDT is opposite to the polarity of the electrode finger of the third IDT adjacent to the second IDT.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to longitudinally coupled resonator typesurface acoustic wave filters. More particularly, the present inventionrelates to longitudinally coupled resonator type surface acoustic wavefilters having balance-unbalance conversion functions.

2. Description of the Related Art

Recently, the sizes and weights of mobile phones have been greatlyreduced. With such a trend, the numbers and sizes of components used inmobile phones have also been reduced while making such components moreand more multifunctional.

With the above-described considerations, various kinds of mobile phoneshave been provided. In these mobile phones, surface acoustic wavefilters are incorporated in the RF stages of the phones to havebalance-unbalance conversion functions, or so-called balun functions.

FIG. 22 is a schematic plan view for illustrating the electrodestructure of a conventional surface acoustic wave filter having abalance-unbalance conversion function.

In this filter, first, second and third IDTs 101, 102 and 103 arearranged in a surface acoustic wave propagating direction. Reflectors104 and 105 are arranged on each side of the surface acoustic wavepropagating direction in the region where the IDTs 101, 102 and 103 arearranged. When a wavelength determined by the electrode finger pitch ofeach of the IDTs 101, 102 and 103 is λI, both a distance between thecenters of the mutually adjacent electrode fingers of the IDTs 101 and102 and a distance between the centers of the mutually adjacentelectrode fingers of the IDTs 102 and 103 are 0.75 λI. The widths of theelectrode fingers 109 and 110 of the ends of the IDT 102 are increasedto reduce spaces between the IDTs. As a result, loss due to theirradiation of a bulk wave can be reduced. In FIG. 22, terminals 106 and107 are balanced signal terminals, and a terminal 108 is an unbalancedsignal terminal.

In such a surface acoustic wave filter having the balance-unbalanceconversion function, regarding propagation characteristics in a passband between the unbalanced signal terminal 108 and the balanced signalterminal 106 and between the unbalanced signal terminal 108 and thebalanced terminal 107, amplitude characteristics need to be equal andpropagating signals need to be 180° out of phase with respect to eachother. The condition in which the amplitude characteristics are equal isreferred to as amplitude balance and the degree at which the propagatingsignals are 180° out of phase with respect to each other is referred toas phase balance.

The amplitude balance and the phase balance are defined as follows whenthe surface acoustic wave filter having the balance-unbalance conversionfunction is regarded as a device having three ports, such as theunbalanced input terminal as port 1 and the balanced output terminals asports 2 and 3.

Amplitude balance=|A|.

A=|20log S21|−|20log S31|.

Phase balance=|B−180|.

B=|∠S21−∠S31|.

In this definition, S21 denotes a transfer factor from the port 1 to theport 2 and S31 denotes a transfer factor from the port 1 to the port 3.

Ideally, in the pass band of the filter, the amplitude balance needs tobe 0 dB and the phase balance needs to be 0°. However, with the use ofthe structure shown in FIG. 22, when intending to obtain a filter havinga balance-unbalance conversion function, since the IDT 102 has an oddnumber of electrode fingers, the number of electrode fingers connectedto the balanced signal terminal 106 increases by one more than thenumber of electrode fingers connected to the balanced signal terminal107. As a result, there is a problem in that the degree of balance isdeteriorated. The higher the central frequency of the filter is, themore noticeable the deterioration. Thus, like DCS filters and PCSfilters, in a filter having a central frequency near 1.9 GHz, sufficientbalance cannot be obtained.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a longitudinally coupled resonator typesurface acoustic wave filter having a balance-unbalance conversionfunction, in which the amplitude balance and the phase balance aregreatly improved. In addition, preferred embodiments of the presentinvention provide a communication apparatus including such a novellongitudinally coupled resonator type surface acoustic wave filter.

According to a preferred embodiment of the present invention, alongitudinally coupled resonator type surface acoustic wave filterhaving a balance-unbalance conversion function includes a piezoelectricsubstrate, first, second and third IDTs arranged on the piezoelectricsubstrate in a surface acoustic wave propagating direction, the secondIDT being positioned between the first and third IDTs and having an evennumber of electrode fingers.

According to another preferred embodiment of the present invention, alongitudinally coupled resonator type surface acoustic wave filterhaving a balance-unbalance conversion function includes first-stage andsecond-stage longitudinally coupled resonator surface acoustic wavefilters longitudinally coupled to each other, each of the first-stageand second-stage filters having a piezoelectric substrate and first,second and third IDTs arranged on the piezoelectric substrate in asurface acoustic wave propagating direction, an unbalanced signalterminal connected to one end of the second IDT of the first-stagelongitudinally coupled resonator surface acoustic wave filter, a firstbalanced signal terminal connected to one end of the second IDT of thesecond-stage longitudinally coupled resonator surface acoustic wavefilter, a second balanced signal terminal connected to the other end ofthe second IDT of the second-stage filter, a first signal lineconnecting one end of the first IDT of the first-stage longitudinallycoupled resonator surface acoustic wave filter and one end of the firstIDT of the second-stage longitudinally coupled resonator surfaceacoustic wave filter, and a second signal line connecting one end of thethird IDT of the first-stage longitudinally coupled resonator surfaceacoustic wave filter and one end of the third IDT of the second-stagelongitudinally coupled resonator surface acoustic wave filter. In thisfilter, an electric signal propagating through the first signal line is180° out of phase with an electric signal propagating through the secondsignal line.

In the filter according to this preferred embodiment of the presentinvention, the second IDT of at least one of the first-stage filter andthe second-stage filter may have an even number of electrode fingers.

According to a third preferred embodiment of the present invention, acommunication apparatus including the longitudinally coupled resonatortype surface acoustic wave filter according to the above-describedpreferred embodiment of the present invention.

The above and other elements, characteristics, features, and advantagesof the present invention will be clear from the following detaileddescription of preferred embodiments of the present invention inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic plan view for illustrating a longitudinallycoupled resonator type surface acoustic wave filter according to a firstpreferred embodiment of the present invention;

FIG. 2 shows a schematic plan view for illustrating the electrodestructure of a longitudinally coupled resonator type surface acousticwave filter presented as a prerequisite for the first preferredembodiment of the present invention;

FIG. 3 shows a schematic plan view of a longitudinally coupled resonatortype surface acoustic wave filter used as another prerequisite for thefirst preferred embodiment of the present invention;

FIG. 4 shows a graph illustrating the relationship between amplitudebalance and frequency characteristics of each of the first preferredembodiment of the present invention and a conventional device;

FIG. 5 shows a graph illustrating the relationship between phase balanceand frequency characteristics of each of the first preferred embodimentof the present invention and the conventional device;

FIG. 6 shows a schematic plan view for illustrating a longitudinallycoupled resonator type surface acoustic wave filter as a first modifiedexample of the first preferred embodiment of the present invention;

FIG. 7 shows a schematic plan view for illustrating a longitudinallycoupled resonator type surface acoustic wave filter as a second modifiedexample of the first preferred embodiment of the present invention;

FIG. 8 shows a schematic plan view for illustrating a longitudinallycoupled resonator type surface acoustic wave filter as a third modifiedexample of the first preferred embodiment of the present invention;

FIG. 9 shows a schematic plan view for illustrating a longitudinallycoupled resonator type surface acoustic wave filter as a fourth modifiedexample of the first preferred embodiment of the present invention;

FIG. 10 shows a schematic plan view for illustrating a longitudinallycoupled resonator type surface acoustic wave filter as a fifth modifiedexample of the first preferred embodiment of the present invention;

FIG. 11 shows a schematic plan view for illustrating a longitudinallycoupled resonator type surface acoustic wave filter according to asecond preferred embodiment of the present invention;

FIG. 12 shows a schematic plan view for illustrating a surface acousticwave filter as a modified example of the second preferred embodiment ofthe present invention;

FIG. 13 shows a graph illustrating the relationship between amplitudebalance and frequency characteristics of each of the second preferredembodiment and the conventional device;

FIG. 14 shows a graph illustrating the relationship between phasebalance and frequency characteristics of each of the second preferredembodiment and the conventional device;

FIG. 15 shows a schematic plan view for illustrating the conventionalfilter prepared for a comparison with the second preferred embodiment ofthe present invention;

FIG. 16 shows a schematic plan view for illustrating another modifiedexample of the second preferred embodiment of the present invention;

FIG. 17 shows a schematic plan view for illustrating a longitudinallycoupled resonator type surface acoustic wave filter according to a thirdpreferred embodiment of the present invention;

FIG. 18 shows a graph illustrating the relationship between amplitudebalance and frequency characteristics of each of the third preferredembodiment of the present invention and the conventional device;

FIG. 19 shows a graph illustrating the relationship between phasebalance and frequency characteristics of each of the third preferredembodiment of the present invention and the conventional device;

FIG. 20 shows a schematic plan view for illustrating a longitudinallycoupled resonator type surface acoustic wave filter according to afourth preferred embodiment of the present invention;

FIG. 21 shows a schematic block diagram for illustrating a communicationapparatus as an example of an apparatus including the longitudinallycoupled resonator type surface acoustic wave filter according topreferred embodiments of the present invention;

FIG. 22 shows a schematic plan view for illustrating the conventionallongitudinally coupled resonator type surface acoustic wave filter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to the drawings, the present invention will be clarifiedby describing preferred embodiments of a longitudinally coupledresonator type surface acoustic wave filter according to the presentinvention.

FIG. 1 shows a schematic plan view for illustrating the longitudinallycoupled resonator type surface acoustic wave filter, preferably for useas a PCS reception filter, according to a first preferred embodiment ofthe present invention. In a longitudinally coupled resonator typesurface acoustic wave filter 200, there is provided an electrodestructure shown in FIG. 1 on a piezoelectric substrate 200A. A 40±5°Y-cut X-propagation LiTaO₃ substrate is preferably used as thepiezoelectric substrate 200A.

On the piezoelectric substrate, first, second and third IDTs 201, 202and 203 are arranged in a surface acoustic wave propagating direction.Reflectors 204 and 205 are arranged in the surface wave propagatingdirection on the right and left of the region where the IDTs 201, 202and 203 are arranged. The IDTs 201, 202 and 203 and the reflectors 204and 205 are preferably made of Al.

In other words, the longitudinally coupled resonator type surfaceacoustic wave filter 200 is a longitudinally coupled resonator typesurface acoustic wave filter having three IDTs.

In FIG. 1, in order to simplify the illustration, the number ofelectrodes shown in the figure is less than the number of electrodes tobe actually included in the device.

One end of each of the IDTs 201 and 203 is connected to an unbalancedsignal terminal 212. One end of the IDT 202 is connected to a balancedsignal terminal 210, and the other end thereof is connected to abalanced signal terminal 211.

The detailed design of an example of the present preferred embodiment ofthe longitudinally coupled resonator type surface acoustic wave filter200 will be presented below.

Electrode finger cross width W=78.8 λI.

The number of the electrode fingers of each of IDTs 201 and 203=24.

The number of the electrode fingers of IDT 202=40.

IDT wavelength λI=2.03 μm.

The wavelength λR of each of reflectors 204 and 205=2.05 μm.

The number of the electrode fingers of each of the reflectors 204 and205=100.

IDT Gap=0.77 λI. In this case, the gap between the IDTs is preferablysubstantially equivalent to a distance between the centers of themutually adjacent electrode fingers of the IDTs.

IDT-reflector gap=0.55 λR. The gap between an IDT and a reflector ispreferably substantially equivalent to a distance between the centers ofthe mutually adjacent electrode fingers of the IDT and the reflector.

IDT Duty=0.60.

Reflector Duty=0.60.

Electrode-film thickness=0.08 λI.

In addition, as shown in FIG. 1, the widths of the electrode fingers 206and 207 on each side of the IDT 202 are preferably broader than those ofthe remaining electrode fingers. With this arrangement, the space in thegap between the IDTs is reduced.

As the characteristics of the first preferred embodiment of the presentinvention, the total number of the electrode fingers of the IDT 202arranged in the center is preferably an even number and an electrodefinger 201 a of the IDT 201 adjacent to the IDT 202 is arranged todefine a signal electrode, whereas an electrode finger 203 a of the IDT203 adjacent to the IDT 202 is arranged to define a ground electrode. Asshown here, of the electrode fingers of the right IDT 201 and the leftIDT 203, the electrode fingers 201 a and 203 a adjacent to the centralsecond IDT 202 have opposite polarities. The reason for this will bedescribed below with reference to FIGS. 2 and 3.

In the conventional longitudinally coupled resonator type surfaceacoustic wave filter 100 shown in FIG. 22, when one of the electrodefingers of the central IDT 102 is removed, as shown in FIG. 2, thenumber of the electrode fingers of the central IDT 102A is an evennumber. On the other hand, since a distance A between the IDT 102A andthe IDT 103 is increased by about 0.5 λI, loss due to the irradiation ofa bulk wave is increased.

Thus, as shown in FIG. 3, there can be considered a structure in whichthe third IDT 103 is shifted by about 0.5 λI toward the IDT 102A.However, in the structure shown in FIG. 3, the IDT 101 and the IDT 103are 180° out of phase with each other.

Therefore, in this preferred embodiment of the present invention, asshown in FIG. 1, by having the IDT 201 reversed from the IDT 203, theIDT 201 and the IDT 203 are in phase with each other.

FIG. 4 shows a graph illustrating the relationship between amplitudebalance and frequencies in the longitudinally coupled resonator typesurface acoustic wave filter according to the first preferred embodimentof the present invention. FIG. 5 shows a graph illustrating therelationship between phase balance and frequencies in the same filter.In FIGS. 4 and 5, solid lines show the results of the first preferredembodiment of the present invention. In addition, for comparison, inFIGS. 4 and 5, broken lines show the characteristics of the conventionalfilter shown in FIG. 22.

In this case, the conventional filter has the same detailed design asthat of the first preferred embodiment, except that one of the electrodefingers of the central IDT in the conventional filter is removed.

A pass-band frequency range used in the PCS reception filter is betweenabout 1930 MHz to about 1990 MHz. As seen in FIG. 4, in the frequencyband range, the maximum amplitude balance is 3.2 dB in the conventionalfilter, whereas it is about 2.7 dB in the first preferred embodiment ofthe present invention. That is, the amplitude balance is improved byabout 0.5 dB in this preferred embodiment of the present invention.Moreover, as shown in FIG. 5, whereas the maximum phase balance in theconventional filter is 21°, the maximum phase balance in the firstpreferred embodiment of the present invention is about 17°. That is,obviously, the phase balance is improved by about 4° in this preferredembodiment of the present invention.

As mentioned above, in this preferred embodiment of the presentinvention, the total number of the electrode fingers of the centralsecond IDT 202 is an even number and the polarity of the electrodefinger of the first IDT 201 adjacent to the central second IDT 202 isreversed from the polarity of the electrode finger of the third IDT 203adjacent to the IDT 202. Thus, as compared with the conventional filter,obviously, the first preferred embodiment of the present inventionprovides a longitudinally coupled resonator type surface acoustic wavefilter having a balance-unbalance conversion function, in which theamplitude balance and the phase balance are both greatly improved.

This preferred embodiment preferably uses a 40±5° Y-cut X-propagationLiTaO₃ substrate. However, the substrate used in the present inventionis not restricted to the LiTaO₃ substrate. Any other piezoelectricsubstrate may be used such as a 64 to 72° Y-cut X-propagation LiNbO₃substrate or a 41° Y-cut X-propagation LiNbO₃ substrate, for example.Other suitable substrates may also be used.

In addition, in the first preferred embodiment of the present invention,a balanced signal is obtained from the central second IDT. However, asshown in FIG. 6, the balanced signal may be obtained from each of thefirst IDT 201 and the third IDT 203 on each side of the central IDT 202.In FIG. 6, terminals 213 and 214 are balanced signal terminals connectedto the first and third IDTs 201 and 203, respectively, and a terminal215 is an unbalanced signal terminal connected to the central second IDT202.

FIG. 7 shows a schematic plan view for illustrating the electrodestructure of another modified example of the first preferred embodimentof the present invention. As shown in FIG. 7, a surface acoustic waveresonator 216 may be connected between the first and third IDTs 201 and203 and a terminal 212.

FIG. 8 shows a schematic plan view for illustrating the electrodestructure of another modified example of the first preferred embodimentof the present invention. In a longitudinally coupled resonator typesurface acoustic wave filter 217 shown in FIG. 8, the longitudinallycoupled resonator type surface acoustic wave filters 200 according tothe first preferred embodiment are longitudinally connected in atwo-stage structure.

In addition, FIG. 9 shows a schematic plan view for illustrating alongitudinally coupled resonator type surface acoustic wave filter asanother modified example of the first preferred embodiment of thepresent invention. In FIG. 9, the longitudinally coupled resonator typesurface acoustic wave filter 200 of the first preferred embodiment islongitudinally connected to a 3-IDT-type longitudinally coupledresonator type surface acoustic wave filter 219 including the centralIDT 218 having an odd number of electrode fingers. In other words, inthe multi-stage longitudinally coupled resonator type surface acousticwave filters, even when at least only one-stage filter is defined by thelongitudinally coupled resonator type surface acoustic wave filter 200of the first preferred embodiment, as in the first preferred embodiment,greatly improved amplitude and phase balances are achieved.

FIG. 10 shows a schematic plan view for illustrating the electrodestructure of another modified example of the longitudinally coupledresonator type surface acoustic wave filter of the first preferredembodiment. In a longitudinally coupled resonator type surface acousticwave filter 220 shown in FIG. 10, narrow pitch electrode finger sectionsN1 to N4 are disposed in first, second and third IDTs 221, 222 and 223.Specifically, in the IDT 221, the narrow pitch electrode finger sectionN1, is arranged such that the electrode finger pitches of some portionsfrom the end portion of the IDT 222 are narrower than the electrodefinger pitches of the remaining portions of the IDT 221. Similarly, oneach end portion of the IDT 222, the narrow pitch electrode fingersections N2 and N3 are provided. In addition, in the IDT 223, on theside adjacent to the IDT 222, the narrow pitch electrode finger sectionN4 is provided. As shown here, in the regions where the IDTs areadjacent to each other, even when disposing the narrow pitch electrodefinger sections having relatively narrow electrode finger pitches, byarranging the remaining portions like the first preferred embodiment,greatly improved amplitude and phase balances are achieved.

FIG. 11 shows a schematic plan view for illustrating the electrodestructure of a longitudinally coupled resonator type surface acousticwave filter according to a second preferred embodiment of the presentinvention.

The second preferred embodiment of the present invention relates to anAMPS reception filter.

Similar to the first preferred embodiment, the electrode structure shownin FIG. 11 is preferably formed on a 40±5° Y-cut X-propagation LiTaO₃substrate to constitute a longitudinally coupled resonator type surfaceacoustic wave filter 300 of the second preferred embodiment of thepresent invention.

In the longitudinally coupled resonator type surface acoustic wavefilter 300, a first longitudinally coupled resonator type surfaceacoustic wave filter 301 and a second longitudinally coupled resonatortype surface acoustic wave filter 302 are longitudinally connected toeach other. The first and second longitudinally coupled resonator typesurface acoustic wave filters 301 and 302 preferably have the samestructure.

Similar to the first preferred embodiment, the first-stagelongitudinally coupled resonator type surface acoustic wave filter 301includes first, second and third IDTs 303, 304 and 305 arranged in asurface acoustic wave propagating direction and reflectors 306 and 307,and the second-stage longitudinally coupled resonator type surfaceacoustic wave filter 302 includes first, second and third IDTs 308, 309and 310 in the surface acoustic wave propagating direction andreflectors 311 and 312. The reflectors 306, 307, 311, and 312 aredisposed on the right and left sides of the regions where the IDTs ofthe filters 301 and 302 are arranged. One end of each of the first andthird IDTs 303 and 305 of the first longitudinally coupled resonatortype surface acoustic wave filter 301 is connected to one end of each ofthe first and third IDTs 308 and 310 of the second longitudinallycoupled resonator type surface acoustic wave filter 302 via first andsecond signal lines 316 and 317. One end of the IDT 304 is connected toa terminal 313. One end of the IDT 309 is connected to a terminal 314and the other end thereof is connected to a terminal 315. The remainingends of the IDTs 303, 304 and 305 and 308, 309 and 310 are connected toground potentials.

The terminals 314 and 315 are balanced signal terminals. The terminal313 is an unbalanced signal terminal. The detailed design of an exampleof the filter 300 will be presented below.

Electrode finger cross width W=49.0 λI.

The number of the electrode fingers of first IDTs 303 and 308=24.

The number of the electrode fingers of second IDTs 304 and 309=34.

The number of the electrode fingers of third IDTs 305 and 310=25.

IDT wavelength λI=4.49 μm.

Reflector wavelength λR=4.64 μm.

The number of reflector electrode fingers=120.

IDT gap=0.79 λI.

IDT-reflector gap=0.47 λR.

IDT duty=0.73.

Reflector duty=0.55.

Electrode film thickness=0.08 λI.

In this preferred embodiment of the present invention, the widths of theelectrode fingers 304 a, 304 b, 309 a, and 309 b on each end of thecentral second IDTs 304 and 309 are broadened to reduce spaces betweenthe adjacent IDTs.

In this preferred embodiment, the number of the electrode fingers ofeach of the IDTs 304 and 309 is preferably an even number as in the caseof the first preferred embodiment. In addition, the electrode fingers303 a and 305 a of the IDTs 303 and 305 adjacent to the IDT 304 and theelectrode fingers 308 a and 310 a of the IDTs 308 and 310 adjacent tothe IDT 309 are arranged to define ground electrodes.

In the first longitudinally coupled resonator type surface acoustic wavefilter 301, the IDTS 303 and 305 are 180° out of phase with the IDT 304.Thus, when the IDTs 303 and 305 are connected in parallel to make thesingle stage filter, it is impossible to obtain filter characteristics.However, simultaneously, in the second longitudinally coupled resonatortype surface acoustic wave filter 302, the IDTs 308 and 310 are also180° out of phase with the IDT 309. As a result, since surface wavespropagating from the IDTs 308 and 310 to the IDT 309 are in phase witheach other, filter characteristics can be obtained by connecting thefirst and second longitudinally coupled resonator type surface acousticwave filters 301 and 302. In other words, an electric signal propagatingthrough a first signal line 316 is 180° out of phase with an electricsignal propagating through a second signal line 317.

FIG. 13 shows the relationship between amplitude balance and frequenciesin the second preferred embodiment of the present invention. FIG. 14shows the relationship between phase balance and frequencies. In FIGS.13 and 14, solid lines indicate the results of the second preferredembodiment and broken lines indicate the results of a conventionaldevice shown in FIG. 15, which is prepared for comparison.

In a conventional longitudinally coupled resonator type surface acousticwave filter 401 shown in FIG. 15, the numbers of the electrode fingersof first, second and third IDTs 402, 403 and 404 are preferably the sameas those of the second preferred embodiment, except that the number ofthe electrode fingers of the first IDT is preferably 24, the number ofthe electrode fingers of the second IDT is preferably 35, and the numberof the electrode fingers of the third IDT is preferably 24.

A pass-band frequency range in the AMPS reception filter is betweenapproximately 860 MHz and approximately 894 MHz.

In the frequency range, the maximum amplitude balance is 1.9 dB in theconventional filter, whereas it is about 0.9 dB in the second preferredembodiment of the present invention. Thus, obviously, the amplitudebalance is improved by about 1.0 dB in the second preferred embodiment.

In addition, whereas the maximum phase balance is 17° in theconventional filter, it is about 8° in the second preferred embodiment.The phase balance is improved by about 9° in the second preferredembodiment of the present invention.

As mentioned above, the reason that the amplitude and phase balances areimproved is as follows. The second IDT has an even number of electrodefingers and the electric signal propagating through the first signalline 316 is 180° out of phase with the electric signal propagatingthrough the second signal line 317. As a result, the electrode fingersof the first and third IDTs adjacent to the central second IDT can beboth arranged to define ground electrodes, and a distance B (see FIG.11) between the electrode finger 309 a connected to the terminal 315 andthe electrode signal finger 308 b of the IDT 308 is substantially equalto a distance C (see FIG. 11) between the electrode finger 309 bconnected to the terminal 314 and the signal electrode finger 310 b ofthe IDT 310. In contrast, in the conventional device shown in FIG. 15,distances D and E between electrode fingers, specified in the figure,between the adjacent IDTs in the second longitudinally coupled resonatortype surface acoustic wave filter are differentiated by about 0.5 λI.

In the second preferred embodiment, the electrode fingers of the rightfirst IDT and the left third IDT adjacent to the second IDT are groundelectrodes. However, even when these electrode fingers constitute signalelectrodes, the same advantages can be obtained.

As mentioned above, in the second preferred embodiment, the first andsecond longitudinally coupled resonator type surface acoustic wavefilters 301 and 302 including the second IDTs having the even number ofelectrode fingers are longitudinally connected in the two-stagestructure. Additionally, the electric signal propagating through thefirst signal line 316 is 180° out of phase with the electric signalpropagating through the second signal line 317. With this arrangement,the polarity of the electrode fingers of the central second IDT can bethe same as the polarities of the electrode fingers of the first andthird IDTs adjacent to the second IDT. Both of the amplitude balance andthe phase balance are also greatly improved.

Also, in the case where narrow pitch electrode fingers are used at theportion where two IDTs are adjacent to each other in the structure shownin FIG. 11, with respect to the second preferred embodiment, as shown inFIG. 10, the characteristics which indicates improved balance can beobtained.

FIG. 12 shows a schematic plan view for illustrating a modified exampleof the second preferred embodiment of the present invention. Thismodified example is differentiated from the second preferred embodimentin that, in the arrangement shown in FIG. 11, the first and secondsurface acoustic wave filters 301 and 302 are symmetrical with respectto the axis Z in FIG. 12. However, in the modified example shown in FIG.12, the surface acoustic wave filters 302 and 302 are symmetrical withrespect to the point Y in the figure, that is, the center of the entireelectrode structure including the surface acoustic wave filters 301 and302.

FIG. 16 shows a schematic plan view for illustrating another example ofthe second preferred embodiment of the present invention. In thismodified example, first and second longitudinally coupled resonator typesurface acoustic wave filters 301 and 321 are longitudinally connectedto each other. Unlike the second preferred embodiment, in thesecond-stage longitudinally coupled resonator type surface acoustic wavefilter 321, the central second IDT 322 is preferably split into twoparts. The remaining parts are preferably substantially the same asthose in the second preferred embodiment. Thus, the same referencenumerals as those used in the second embodiment are given to the sameparts in the modified example, and the explanation thereof will beomitted.

The second IDT 322 has an interdigital electrode 322 a, and twointerdigital electrodes 322 b and 322 c arranged such that the electrodefingers of the interdigital electrode 322 a are interdigitated with theelectrode fingers of the interdigital electrodes 322 b and 322 c. Inother words, one of a pair of the interdigital electrodes defining thesecond IDT 322 is split into the two interdigital electrodes 322 b and322 c, which are connected to balanced signal terminals 314 and 315. Inthis case, in the first-stage longitudinally coupled resonator typesurface acoustic wave filter 301, the IDT 303 is 180° out of phase withthe IDT 305. In the second-stage longitudinally coupled resonator typesurface acoustic wave filter 321, the first IDT 308 is 180° out of phasewith the second IDT 310. With this arrangement, an electric signalpropagating through a signal line 316 is 180° out of phase with anelectric signal propagating through a signal line 317. Thus, thepolarities of the electrode fingers of the IDT 308 and the IDT 310adjacent to the IDT 322 are the same. As a result, the amplitude andphase balances in this filter are greatly improved as with the secondpreferred embodiment of the present invention.

FIG. 17 shows a schematic plan view for illustrating the electrodestructure of a longitudinally coupled resonator type surface acousticwave filter according to a fourth preferred embodiment of the presentinvention.

A longitudinally coupled resonator type surface acoustic wave filter 600of the fourth preferred embodiment preferably has the same structure asthat of the second preferred embodiment, except for the followingpoints.

In this preferred embodiment, in first-stage and second-stagelongitudinally coupled resonator type surface acoustic wave filters 601and 602, the numbers of the electrode fingers of first, second and thirdIDTs are determined, in which the number of the electrode fingers ofeach of the first IDTs 603 and 608 is preferably 24, the number of theelectrode fingers of each of the second IDTs 604 and 609 is preferably35, and the number of the electrode fingers of each of the third IDTs605 and 610 is preferably 24.

In the fourth preferred embodiment, characteristically, the polarity ofan electrode finger 603 a of the first IDT 603 adjacent to the IDT 604is opposite to the polarity of an electrode finger 605 a of the thirdIDT 605 adjacent to the IDT 604. In addition, the polarity of anelectrode finger 608 a of the first IDT 608 adjacent to the IDT 609 isopposite to an electrode finger 610 a of the third IDT 610 adjacent tothe IDT 609.

When considering only the first-stage longitudinally coupled resonatortype surface acoustic wave filter 601, each of the IDTs 603 and 605 is180° out of phase with the IDT 604. Thus, it is impossible to obtainfilter characteristics only by the single-stage filter structure inwhich the IDTs 603 and 605 are connected in parallel to each other.However, simultaneously, in the second-stage longitudinally coupledresonator type surface acoustic wave filter 602, each of the IDTs 608and 610 is 180° out of phase with the IDT 609. As a result, sincesurface acoustic waves propagating from the IDTs 608 and 610 to the IDT609 are in phase with each other, when the first-stage filter and thesecond-stage filter are connected, filter characteristics can beobtained.

A terminal 615 is an unbalanced signal terminal and terminals 616 and617 are balanced signal terminals.

In FIG. 18, a solid line shows the relationship between amplitudebalance and frequency characteristics of the longitudinally coupledresonator type surface acoustic wave filter of the fourth preferredembodiment. In FIG. 19, a solid line shows the relationship betweenphase balance and frequency characteristics of the surface acoustic wavefilter. For comparison, broken lines show the result of the conventionalfilter shown in FIG. 15.

As found in FIG. 19, the maximum amplitude balance in a pass band in theAMPS reception filter is 1.9 dB in the conventional filter, whereas itis about 1.2 dB in the fourth preferred embodiment of the presentinvention. Thus, the amplitude balance is improved by about 0.7 dB.Additionally, as shown in FIG. 19, whereas the maximum phase balance is17° in the conventional filter, it is about 11° in the fourth preferredembodiment. Thus, the phase balance is improved by about 6° in thefourth preferred embodiment.

In other words, in the fourth preferred embodiment, the first-stage andfirst-stage longitudinally coupled resonator type surface acoustic wavefilters having three IDTs are longitudinally connected to each other. Inaddition, the polarities of the electrode fingers of the first and thirdIDTs adjacent to the central second IDT are opposite to each other. Withthis arrangement, the amplitude balance and the phase balance aregreatly improved.

FIG. 20 shows a schematic plan view for illustrating the electrodestructure of the longitudinally coupled resonator type surface acousticwave filter according to a fifth preferred embodiment of the presentinvention. The fifth preferred embodiment uses 3-IDT-type longitudinallycoupled resonator type surface acoustic wave filters 901, 902, 903 and904 having first, second and third IDTs. In each of the longitudinallycoupled resonator type surface acoustic wave filters 901, 902, 903 and904, the central second IDT has even electrode fingers, and thepolarities of the electrode fingers of the right and left IDTs adjacentto the central second IDT are the same. For example, in thelongitudinally coupled resonator type surface acoustic wave filter 901,the electrode finger 907 a of the first IDT 907 and the electrode finger909 a of the third IDT 909 adjacent to the second IDT 908 have the samepolarity.

Although the longitudinally coupled resonator type surface acoustic wavefilters 901, 902 and 903 preferably have the same structure, only thelongitudinally coupled resonator type surface acoustic wave filter 902has the second IDT 910 reversed.

The reference numeral 911 denotes an unbalanced signal terminal and thereference numerals 912 and 913 denote balanced signal terminals.

In each of the first, second, third and fourth longitudinally coupledresonator type surface acoustic wave filters 901, 902, 903 and 904, thefirst IDT is 180° out of phase with the third IDT. For example, in thelongitudinally coupled resonator type surface acoustic wave filter 901,the first IDT 907 is 180° out of phase with the third IDT 909.

In the fifth preferred embodiment, similar to the second, third andfourth preferred embodiments, the amplitude balance and the phasebalance in the filter are greatly improved. Moreover, in the fifthpreferred embodiment, output impedance substantially quadruples.

FIG. 21 shows a schematic plan view for illustrating a communicationapparatus 60 incorporating the longitudinally coupled resonator typesurface acoustic wave filter according to other preferred embodiments ofthe present invention.

In FIG. 21, an antenna 61 is connected to a duplexer 62. A surfaceacoustic wave filter 64 and an amplifier 65 are connected between theduplexer 62 and a mixer 63 of the reception section. An amplifier 67 anda surface acoustic wave filter 68 are connected between the duplexer 62and a mixer 66 of the transmission section. As shown here, when theamplifier 65 is adapted to a balanced signal, the longitudinally coupledresonator type surface acoustic wave filter of various preferredembodiments of the present invention can be suitably used as the surfaceacoustic wave filter 64.

As described above, in the longitudinally coupled resonator type surfaceacoustic wave filter according to various preferred embodiments of thepresent invention, the central second IDT preferably has an even numberof electrode fingers. In addition, the polarities of electrode fingersadjacent to the second IDT in the first and third IDTs disposed on theright-and-left sides of the second IDT are opposite. Thus, the amplitudebalance and the phase balance between the balanced signal terminals aregreatly effectively improved. As a result, preferred embodiments of thepresent invention provide a surface acoustic wave filter having anexcellent balance-unbalance conversion function.

Furthermore, in the longitudinally coupled resonator type surfaceacoustic wave filter according to other preferred embodiments of thepresent invention, first-stage and second-stage longitudinally coupledresonator type surface acoustic wave filters having first, second andthird IDTs are longitudinally connected to each other. One end of thesecond IDT of the first-stage longitudinally coupled resonator typesurface acoustic wave filter is connected to an unbalanced signalterminal. Additionally, both ends of the second IDT of the second-stagelongitudinally coupled resonator type surface acoustic wave filter areconnected to a pair of balanced signal terminals. In this arrangement,an electric signal propagating through a first signal line connectingone end of the first IDT of the first-stage longitudinally coupledresonator type surface acoustic wave filter and one end of the first IDTof the second-stage longitudinally coupled resonator type surfaceacoustic wave filter is 180° out of phase with an electric signalpropagating through a second signal line connecting one end of the thirdIDT of the first-stage longitudinally coupled resonator type surfaceacoustic wave filter and one end of the third IDT of the second-stagelongitudinally coupled resonator type surface acoustic wave filter. As aresult, the polarity of the electrode finger of the second IDT adjacentto each of the first and third IDTs can be the same as the polarity ofthe electrode finger of each of the first and third IDTs adjacent to thesecond IDT. Thus, the amplitude balance and the phase balance in thefilter are effectively improved.

According to this preferred embodiment of the present invention, in atleast one of the first-stage and second-stage longitudinally coupledresonator type surface acoustic wave filters, when the second IDT has aneven number of electrode fingers, the amplitude balance and the phasebalance are improved even more.

In various preferred embodiments of the present invention, thelongitudinally coupled resonator type surface acoustic wave filter hasthe above-described balance-unbalance conversion function and thebalance between the pair of balanced signal terminals is greatlyimproved. Accordingly, when producing a communication apparatusincorporating the longitudinally coupled resonator type surface acousticwave filter, the characteristics of the communication apparatus can begreatly improved while reducing the size of the apparatus.

While the present invention has been described with reference to whatare at present considered to be preferred embodiments, it is to beunderstood that various changes and modifications may be made theretowithout departing from the invention in its broader aspects andtherefore, it is intended that the appended claims cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A longitudinally coupled resonator type surfaceacoustic wave filter having a balance-unbalance conversion function, thefilter comprising: a piezoelectric substrate; and first, second andthird IDTs arranged on the piezoelectric substrate in a surface acousticwave propagating direction, the second IDT being located between thefirst and third IDTs and having an even number of electrode fingers;wherein said second IDT includes two opposed bus bars and said electrodefingers of said second IDT extend from each of said two opposed bus barstoward each other and are interdigitated with each other, said electrodefingers of said second IDT are interdigitated such that no two of theelectrode fingers extending from one of said two opposed bus bars areimmediately adjacent to each other; and electrode fingers of said firstand third IDTs adjacent to the second IDT have opposite polarities.
 2. Alongitudinally coupled resonator type surface acoustic wave filteraccording to claim 1, wherein the piezoelectric substrate is made of oneof LiTaO3 and LiNbO3.
 3. A longitudinally coupled resonator type surfaceacoustic wave filter according to claim 1, further comprising reflectorsarranged in the surface wave propagating direction on the right and leftof the region where the first, second and third IDTs are arranged.
 4. Alongitudinally coupled resonator type surface acoustic wave filteraccording to claim 1, wherein widths at the electrode fingers on eachside of the second IDT are larger than those of the remaining electrodefingers.
 5. A communication apparatus comprising the longitudinallycoupled resonator type surface acoustic wave filter according toclaim
 1. 6. A longitudinally coupled resonator type surface acousticwave filter according to claim 1, further comprising a surface acousticwave resonator connected between the first and third IDTs and aterminal.
 7. A longitudinally coupled resonator type surface acousticwave filter according to claim 1, wherein each of the first, second andthird IDTs include narrow pitch electrode finger sections that arerelatively narrower than others of the electrode finger sectionsincluded in the first, second and third IDTs.
 8. A longitudinallycoupled resonator type surface acoustic wave filter having abalance-unbalance conversion function, the filter comprising:first-stage and second-stage longitudinally coupled resonator typesurface acoustic wave filters longitudinally coupled to each other, eachof the first-stage longitudinally coupled resonator type surfaceacoustic wave and the second-stage longitudinally coupled resonator typesurface acoustic wave filter including a piezoelectric substrate andfirst, second and third IDTs arranged on the piezoelectric substrate ina surface acoustic wave propagating direction, said second-stagelongitudinally coupled resonator type surface acoustic wave filterincluding two opposed bus bars and electrode fingers extending in alongitudinally direction of the electrode fingers from each of said twoopposed bus bars and being interdigitated with each other; an unbalancedsignal terminal connected to one end of the second IDT of thefirst-stage longitudinally coupled resonator type surface acoustic wavefilter; a first balanced signal terminal connected to one of said twoopposed bus bars of the second IDT of the second-stage longitudinallycoupled resonator type surface acoustic wave filter; a second balancedsignal terminal connected to the other of said two opposed bus bars ofthe second IDT of the second-stage longitudinally coupled resonator typesurface acoustic wave filter; a first signal line connecting one end ofthe first IDT of the first-stage longitudinally coupled resonator typesurface acoustic wave filter and one end of the first IDT of thesecond-stage longitudinally coupled resonator type surface acoustic wavefilter; and a second signal line connecting one end of the third IDT ofthe first-stage longitudinally coupled resonator type surface acousticwave filter and one end of the third IDT of the second-stagelongitudinally coupled resonator type surface acoustic wave filter;wherein an electric signal propagating through the first signal line is180° out of phase with an electric signal propagating through the secondsignal line.
 9. A communication apparatus comprising the longitudinallycoupled resonator type surface acoustic wave filter according to claim8.
 10. A longitudinally coupled resonator type surface acoustic wavefilter according to claim 8, wherein the second IDT of at least one ofthe first-stage longitudinally coupled resonator type surface acousticwave filter and the second-stage longitudinally coupled resonator typesurface acoustic wave filter has an even number of electrode fingers.11. A longitudinally coupled resonator type surface acoustic wave filteraccording to claim 8, wherein the piezoelectric substrate of each of thefirst-stage and second-stage longitudinally coupled resonator typesurface acoustic wave filters is made of one of LiTaO3 and LiNbO3.
 12. Alongitudinally coupled resonator type surface acoustic wave filteraccording to claim 8, wherein each of first-stage and second-stagelongitudinally coupled resonator type surface acoustic wave filtersfurther comprises reflectors arranged in the surface wave propagatingdirection on the right and left of the region where the first, secondand third IDTs are arranged.
 13. A longitudinally coupled resonator typesurface acoustic wave filter according to claim 8, wherein in each ofthe first-stage and second-stage longitudinally coupled resonator typesurface acoustic wave filters, widths of the electrode fingers on eachside of the second IDT are larger than those of the remaining electrodefingers.
 14. A longitudinally coupled resonator type surface acousticwave filter according to claim 8, wherein in each of the first-stage andsecond-stage longitudinally coupled resonator type surface acoustic wavefilters, electrode fingers of said first and third IDTs adjacent to thesecond IDT have opposite polarities.
 15. A longitudinally coupledresonator type surface acoustic wave filter according to claim 8,wherein each of the first-stage and second-stage longitudinally coupledresonator type surface acoustic wave filters further comprises a surfaceacoustic wave resonator connected between the first and third IDTs and aterminal.
 16. A longitudinally coupled resonator type surface acousticwave filter according to claim 8, wherein in each of the first-stage andsecond-stage longitudinally coupled resonator type surface acoustic wavefilters, each of the first, second and third IDTs include narrow pitchelectrode finger sections that are relatively narrower than others ofthe electrode finger sections included in the first, second and thirdIDTs.
 17. A longitudinally coupled resonator type surface acoustic wavefilter according to claim 8, wherein the first-stage and second-stagelongitudinally coupled resonator type surface acoustic wave filters havethe same structure.
 18. A longitudinally coupled resonator type surfaceacoustic wave filter according to claim 8, wherein in each of thefirst-stage and second-stage longitudinally coupled resonator typesurface acoustic wave filters, the finger electrodes of the first andthird IDTs that are adjacent to the central second IDT are arranged todefine ground electrodes.
 19. A longitudinally coupled resonator typesurface acoustic wave filter according to claim 8, wherein in each ofthe first-stage and second-stage longitudinally coupled resonator typesurface acoustic wave fitters, the polarities of the electrode fingersof the second IDT adjacent to the first and third IDTs are the same asthe polarities of electrode fingers of the first and third IDTs adjacentto the second IDT.
 20. A longitudinally coupled resonator type surfaceacoustic wave filter according to claim 8, wherein the first-stage andsecond-stage longitudinally coupled resonator type surface acoustic wavefilters are symmetrical to each other.
 21. A longitudinally coupledresonator type surface acoustic wave filter having a balance-unbalanceconversion function, the filter comprising: a piezoelectric substrate;first, second and third IDTs arranged on the piezoelectric substrate ina surface acoustic wave propagating direction, the second IDT beinglocated between the first and third IDTs and having an even number ofelectrode fingers; and a surface acoustic wave resonator connectedbetween the first and third IDTs and a terminal; wherein said second IDTincludes two opposed bus bars and said electrode fingers of said secondIDT extend from each of said two opposed bus bars toward each other andare interdigitated with each other, said electrode fingers of saidsecond IDT are interdigitated such that no two of the electrode fingersextending from one of said two opposed bus bars are immediately adjacentto each other.
 22. A communication apparatus comprising thelongitudinally coupled resonator type surface acoustic wave filteraccording to claim 21.